OK so my friend with the 351 i sold him is having problems with it he has changed the cam put aluminum heads (pro comp) eddy RPM air gap his carb is pieced together it is a vacuum secondary dual feed, not double pumper, pro comp main body with adjustable air bleeds and adjustable secondary, metering plates are pro comp and i think bowls are pro comp. He didnt buy this as a unit but a piece at a time and Mallory sign. with the timing locked out at 36 total. At about 4500 rpm it starts misfiring and doesn't stop until he gets the rpm back down. is it timing or maybe the secondaries opening or not, is it a vacuum leak? could be a weak or broken valve spring I think he's checked to see if any are broken. I'm lost any other suggestions

It could Be a Lean misfire . He has to read the color of the Plugs all of them . If they are white could be carb Lean or Low Fuel Pressure . Also do a Compression Test on all the Cylinders when the Plugs are all out . Make sure none of the Cylinders are weak .

But right off the top I really am thinking it's a Bad Wire or cracked Plug . Some Times you can see the Voltage jumping ground if you park the car in the darkest spot at night and open the hood . Also one Plug much darker then the others if the Cylinder Compression test is good . Could be a Bad Spark Plug or Wire on that Cylinder .

he's been reading the #1 plug I'm sure he's not reading all, he's changed plugs a couple times I think they were all black. I'll tell him to look at the wires and check those plugs again thanks for the help Tango

It could be something as simple as a Spark Plug wire that burnt on a Header tube . Or a Spark plug that may have cracked when installed . Black is rich mixture or weak Spark . A Wet Black Plug way over rich or Low or Dead Cylinder compression Due to bad valve or ring seal / bad valve adjustment to tight . If he has good Compression but has a Plug way Lean or wet with over fueling Loose Valves adjustment or Worn Camshaft Lobes .

Best way to read Plugs is with a New set . Warm up the Engine . Make two passes through the gears . Very soon after with out Long Idling . Pull the car off the Road and pull all the Plugs out . Then Read them .

good stuff Tango, with the description you gave it is possible that he has adjusted his valves to tight. I'll see him this weekend and maybe I can get him to do a compression test as well as the valve adjustment. My concern was that this motor had around 40,000 on it and is a completly stock motor after a season of 1/4 mile runs he put the new heads on, if the rings were week at all those new heads were going to be putting then to the test.

With 40K even a bit more Miles . That short Block is up for the task . If he has the Valves to tight it could cause a Valve to be off it's seat and give Low to Zero Compression on that Cylinder . When he does that Compression test . If he has a Cylinder thats Low . Have him back off both Rocker Nuts so the Valve stay Closed . It that bump the Compression up it was the valve adjustment . If not have him squirt some oil in that Cylinder and check it again . If it goes up then It bad ring seal in that Cylinder . If all the cylinders are Low that a Bad Timing Chain / Valve Timing .